The present invention relates in general to infrared (IR) sensing devices, and more particularly to a self-calibrating IR sensing device.
IR sensors are used to measure temperatures of remote objects by detecting the infrared radiation emitted from the target object. In a typical configuration, IR radiation enters the housing in which the sensor element is located through an IR transparent window and impinges upon the sensor. The temperature is typically measured by detecting the IR radiation and determining its effect on a thermally isolated radiation absorbing area of the sensor. When the surface of the window becomes contaminated by particles or residues that absorb IR radiation, incoming IR radiation is absorbed such that the amount of IR radiation passing through the window becomes attenuated. The IR transmission capability of the window is therefore degraded by the presence of IR-absorbing contaminants. Such degradation results in erroneous temperature readings by the sensor. Accordingly, what is needed in the art is the ability to easily counter the attenuation effects of IR-absorbing contaminants deposited on the transmission window so as to accurately detect and measure the IR radiation received from a target object.
The present invention provides a self-calibrating infrared (IR) sensing device having the capability to measure the attenuation effects of surface contamination on the transmission window and to adjust the gain of the sensor accordingly so as to counter the attenuation caused by any IR-absorbing contaminants on the transmission window.
According to the invention, an IR sensing device includes an IR sensor mounted in a housing having an IR transparent window that allows IR radiation originating from outside the housing, e.g., from a target source, to be directed toward the sensor. The radiation detected by the sensor is made up of a combination of the external IR radiation entering the housing through the IR transparent window and the inherent background radiation from the inner surface of the housing. The IR sensor includes a thermally isolated area of material selected for its ability to absorb IR radiation at a specific frequency or at a range of radiation frequencies. The incoming radiation is absorbed, thereby raising the temperature of the IR absorbing material. A temperature measuring unit coupled to the absorbing material measures the increase in temperature and generates a proportional electronic signal indicative of the temperature. The electronic signal is received and processed by a processor.
An IR radiator is also included within the sensing device housing. When activated, the radiator emits IR radiation that passes through the window and is lost to the outside if there is no contamination. If, however, the surface of the window is contaminated, radiation will be absorbed by the contaminants, thereby raising the temperature of the contaminants. This increase in temperature results in a return signal (i.e., IR emissions from the contaminants) that is detected by the sensor. For example, if the radiator is pulsed, the resulting return signal will be in the shape of decaying pulses. The magnitude of the return signal indicates the amount of contaminant on the surface of the window. The return signal is detected and measured by the IR sensor, and is used to signal a warning and/or determine the amount of gain necessary to compensate for the loss of true signal caused by the presence of the contamination. Additionally, IR radiation that falls on the inside of the housing is reflected back to the sensor. If the inside surface of the housing is uncontaminated, the return signal will be as expected. If, however, the housing is internally corroded or contaminated, perhaps indicative of leakage, the return signal will be greater as the contamination heats up and emits IR radiation. Again, the appropriate gain is determined to compensate for any contamination in the housing.
A curved IR reflective mirror is positioned within the sensing device housing to reflect IR radiation from the external target onto the IR sensor. In this arrangement, the radiation from the IR radiator is passed to the window either directly or by reflection via the internal curved mirror. This arrangement has no return signal from the inside of the housing, and thus the task of determining the return signal from the contamination on the window is simplified. Use of the internal mirror also helps focus more of the IR radiation entering the window from the external target onto the IR sensor, therefore increasing the magnitude of the signal from the sensor and increasing the sensitivity of the sensor.
According to an aspect of the invention, an IR sensing device is provided. The IR sensing device includes an IR sensor for detecting IR radiation, wherein the IR sensor generates electrical signals in response to and indicative of any IR radiation detected by the IR sensor. The sensing device also typically includes an IR opaque enclosure surrounding the IR sensor, the enclosure having an IR transparent window positioned to allow IR radiation from outside the enclosure to fall on the IR sensor, and an IR radiator positioned within the enclosure, wherein the IR radiator emits IR radiation. In operation, a first portion of a first IR radiation signal generated by the IR radiator directly impinges on the window, and a second portion of the first IR radiation signal is reflected by the mirror element toward the window. At least a portion of each of the first and second portions of the first IR radiation signal is absorbed by IR-absorbing material on the window. In response, the IR-absorbing material emits a return IR radiation signal that is detected by the IR sensor, which generates a first electric signal proportional to the return IR radiation signal. The sensing device also typically includes a processor coupled to the IR sensor for analyzing the electrical signals generated by the IR sensor, wherein the processor analyzes the first electric signal to determine characteristics of the return signal.
According to another aspect of the invention a method of measuring the temperature of a remote target object with an IR sensing device, wherein the IR sensing device includes an IR sensor mounted in a housing, the housing having a window through which IR radiation from a remote target object enters the device and impinges on the IR sensor. The method typically includes the step of generating a first IR radiation signal with an IR radiator located within the housing, wherein a first portion of the first IR radiation signal directly impinges on the window, and reflecting, with an internal mirror, a second portion of the first IR radiation signal toward the window, wherein if there is any IR-absorbing contamination on the window, the contamination absorbs the first and second portions of the first IR radiation signal and emits a return IR radiation signal. The method also typically includes the steps of detecting the return IR radiation signal with the IR sensor, and generating a first electric signal in response to the return IR radiation signal, wherein the first electric signal is proportional to the return signal. The method further typically includes the steps of analyzing the first electric signal with a processor to determine the amount of IR radiation in the return signal, wherein the amount of IR radiation in the return signal is indicative of the amount of IR absorbing contamination on the window. Thereafter a remote IR radiation signal generated by the remote object is detected, wherein the remote IR radiation signal is indicative of the temperature of the remote object, wherein the remote signal is attenuated by a first amount by the IR absorbing contamination before being detected by the IR sensor. The method also typically includes the step of compensating for the first amount of attenuation in the detected remote IR radiation signal so as to accurately determine the temperature of the remote object.
According to yet another aspect of the invention, an Infra Red (IR) sensing device is provided which typically comprises an IR sensing means for detecting IR radiation, wherein the IR sensing means generates an electrical signal responsive to and indicative of the level of IR radiation falling on it; electronic interface means for processing the electrical signal, wherein the interface means generates a signal representative of the IR radiation; and an IR opaque enclosure surrounding the IR sensing means and the electronic interface means, the enclosure including an IR transparent window arranged to allow IR radiation from a remote target object to fall on the IR sensing means. The device also typically comprises a mirror element, located within the enclosure, wherein the mirror element is capable of reflecting IR radiation, and an IR radiating means for emitting IR radiation within the enclosure. In operation, the IR radiating means emits a first IR radiation signal, wherein a first portion of the first IR radiation signal directly impinges on the window, and wherein a second portion of the first IR radiation signal is reflected by the mirror element toward the window, wherein at least a portion of each of the first and second portions of the first IR radiation signal is absorbed by IR-absorbing material on the window. The device also typically includes a first control means for controlling the IR radiation means to generate the first IR signal; a signal processing means for determine the amount of contamination obscuring the IR transparent window by measuring the electrical signal; and a second control means for modifying the characteristics of the electronic interface means to compensate for the amount of contamination.
Reference to the remaining portions of the specification, including the drawings and claims, will realize other features and advantages of the present invention. Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with respect to the accompanying drawings. In the drawings, like reference numbers indicate identical or functionally similar elements.